Electrophotography generally consists of charging an electrophotographic photoreceptor comprising a photoconductor, exposing the photoreceptor to light to form an electrostatic latent image thereon, developing the latent image with a toner composition comprising a binder resin having dispersed therein a colorant, and transferring and fixing the developed toner image onto transfer paper, etc. to obtain a visible image.
Toner compositions widely employed for obtaining a visible image generally comprise a binder resin having dispersed therein a black colorant, such as carbon black. In recent years, color toners comprising a binder resin having dispersed therein a cyan pigment, a magenta pigment or a yellow pigment have been used. These color toners are also employed for obtaining a multicolor image by electrophotographic techniques usually as a combination of color toners of four colors: cyan, magenta, yellow, and black. Where the color toners are applied to an over-head projector (OHP) system, it is required that a color toner image transferred and fixed onto an OHP sheet is projected by transmitted light on a screen to provide a clear color image.
In general, a color toner comprises a binder resin, a colorant, and various additives. Commonly employed binder resins include polystyrene, styrene-(meth)acrylic ester copolymers, styrene-butadiene copolymers, polyester, epoxy resins, and coumaroneindene resins.
A multicolor image is obtained by repetition of exposure to monochromatic light by means of color separation filters, development of each latent image with a corresponding color toner, e.g., a yellow toner, a magenta toner, a cyan toner, and a black toner, and transfer and fixing of each toner image onto transfer paper, etc.
Many techniques concerning color toners have been proposed to date. With respect to magenta color toners, for example, JP-A-51-24234 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a magenta toner containing a quinacridone pigment, and JP-A-59-165069 and JP-A-62-296167 disclose magenta toners containing various monoazo pigments.
However, these conventional color toners have difficulty in maintaining a predetermined quantity of charge in a stable manner against environmental changes from a low-temperature (e.g., 10.degree. C.) and low-humidity (e.g., 20% RH) condition to a high-temperature (e.g., 40.degree. C.) and high-humidity (e.g., 90% RH) condition so that image characteristics are extremely liable to variations depending on environmental conditions.
On the other hand, a multicolor toner image obtained by repetition of development with magenta, cyan, yellow, and black toners is generally demanded to have an image quality close to natural colors. To meet this demand, it is important for each of magenta, cyan, yellow, and black toners to have excellent coloring power and satisfactory color reproducibility, and there has been a need for further improvements.